This section provides background information related to the present disclosure which is not necessarily prior art.
Virtually all motor vehicles are equipped with a coolant pump, commonly referred to as a water pump, to circulate a liquid coolant through the engine cooling circuit for the purpose of controlling thermal transfer from the engine to the coolant for optimized engine operation. In many instances, the water pump is a belt-driven accessory drive arrangement driven off of the engine's crankshaft. Typically, some type of clutch is provided to regulate pump operation and minimize system losses. Recently, many vehicles have been equipped with electric water pumps that can be variable controlled to provide improved pumping efficiency. Many types of electric water pumps are used in vehicular operations, and are typically driven solely in a first or “pumping” direction. Limited rotation in a second direction is sometimes provided to dislodge debris.
A preferred method of controlling a brushless direct current (BLDC) motor is referred to as “sensorless control”, where the position of the rotor relative to the stator is determined by reading the back electromotive force (EMF) generated by the magnets in the rotor passing the coils in the stator. This is preferred because it is less costly than use of sensors to detect the rotor position. The downside of sensorless control is that it limits the minimum speed that a motor can reach in closed loop control while maintaining an ability to read the EMF, which, for example, is typically about 10-15% of the maximum motor speed. A typical water pump operates at a maximum motor speed of about 6000 rpm, and thus, the minimum speed at which the sensorless control in a closed loop arrangement is generally effective is about 600 rpm. The water pump can run with sensorless control at lower speeds, but only in an open loop control arrangement. Unfortunately, without proper feedback to determine the position of the rotor relative to the stator, the pump may lose diagnostic capability (i.e. it cannot verify its operational accuracy) and, therefore, requires additional power to reliably ensure rotation.
Thus a need exists for an electric water pump that can provide a very low flow, while maintaining an ability to utilize sensorless control during the low flow condition, thereby avoiding the power penalty associated with running the pump in an open loop arrangement. The goal is to meet very low flow requirements relative to the maximum speed of the pump without need for expensive sensors, loss of diagnostic feedback and/or higher power consumption associated with conventional open loop control.